Spontaneously adsorbed monolayer films: fabrication, characterization, and application of monolayers of alkanethiol and sulfur-bearing cyclodextrin derivatives

Abstract

<p>Monolayers of n-alkanethiols ( CH[subscript]3(CH[subscript]2)[subscript]nSH, n = 1-17) and sulfur-bearing cyclodextrin derivatives spontaneously adsorbed on Ag and Au have been studied with a variety of surface characterization methods, such as infrared reflection spectroscopy, contact angles, electrochemistry, optical ellipsometry, and scanning tunneling microscopy;Long chain n-alkanethiols monolayers on Ag and Au are insulating to electron transfer and have contact angles indicative of well-ordered hydrocarbon terminated structures. Infrared and contact angle data indicate a different orientation of the methyl group with respect to the surface for chains with odd and even numbers of methylene groups. Compared to monolayers on Au, the alkanethiol monolayers on Ag are oriented more towards the surface normal. The observed odd-even effect of methyl group orientation for these monolayers on Ag is offset by a methylene group from that on Au. The relationships between the structure and packing of the monolayers on Ag and Au and the composition, roughness, and crystallinity of the substrate are also discussed;Monolayers of sulfur-bearing cyclodextrin derivatives on Au and Ag are fabricated by spontaneous adsorption and characterized by the above techniques. Size-selectivity and molecular recognition of the [alpha]- and [beta]-cyclodextrin cavity are shown with our monolayers. Because of molecular recognition, p-nitrophenol is retained preferrentially by the cyclodextrin monolayers over o-nitrophenol. Infrared spectroscopic evidence of the molecular recognition of monolayers is more reproducible than electrochemical evidence. Potential applications of these monolayers are also discussed.</p>